The present invention relates to wireless data communications systems, particularly systems using standardized protocols, and also to wireless systems for locating items.
Wireless communication has been available in varying forms since early in the 20th Century. However, the ability to readily connect through a wireless medium by radio on an anytime-anywhere basis, particularly for data communication purposes, is more recent. The recent ability to connect on anytime-anywhere basis has been facilitated by developments of cellular and Personal Communications Services (PCS) networks into second and third generation digital; the entry of new competitive wireless service providers; vast improvements in digital signal processing; and new commonly-adopted industry Standards such as IEEE Standard 802.11, IEEE Standard 802.16, IEEE Standard 802.20, Wireless Application Protocol (WAP), or the Bluetooth protocol. These developments have resulted in dramatic growth in the infrastructure and capabilities of wireless data communication networks (e.g., wireless local area networks (WLAN), communication products (e.g. cell phones, personal digital assistants (PDA), etc.) and user interest.
Co-pending and co-assigned application Ser. No. 09/528,697, filed Mar. 17, 2000, which is incorporated by reference herein, describes a wireless system which follows the protocol of IEEE Standard 802.11, but which uses a combination of RF Ports and Cell Controllers to perform the functions of Access Points of a classical IEEE 802.11, IEEE Standard 802.16, or IEEE Standard 802.20, data communications system. Lower level protocol functions of the medium access control (MAC) functions are performed by the RF Ports and higher-level protocol functions, including association and roaming functions, are performed by the cell controller. The term “access point” as used herein is intended to include conventional access points, such as those which follow the protocol of IEEE Standard 802.11 and perform all MAC functions, as well as RF Ports operating with cell controllers, as described in the incorporated co-pending application.
The IEEE Standard 802.11 for WLANs specifies a common interface (i.e., the “802.11 MAC Layer”) between devices (e.g., network access points and radios or radio network interface cards), which provides a variety of functions to support the operation a conforming WLAN for data communications. The 802.11 MAC Layer manages and maintains communications between devices by coordinating access to a shared radio channel and utilizing protocols that enhance communications over the wireless medium. The 802.11 MAC Layer uses an 802.11 Physical (PHY) Layer (e.g., as defined by standards such as 802.11b or 802.11a) to perform the tasks of carrier sensing, transmission, and receiving of 802.11 wireless data or message frames. See e.g., http://standards.ieee.org/getieee802/802.11.html.
The primary functions of the 802.11 MAC layer include functions such as “Scanning”, “Probing”, “Request-to send/Clear-to send” (RTS/CTS), and “Power Save/Poll Mode” (PSM). The Scanning function is used when a radio device's network interface card (radio NIC) searches for suitable access points in the WLAN. Each access point periodically broadcasts a radio beacon that contains information about the access point, including service set identifier (SSID), supported data rates, etc. The radio NIC receives the access points' beacons while scanning. The radio NIC may use the information coded in the received beacons along with the beacon signal strengths to compare access points and decide upon which access point to associate with for sending or receiving WLAN data frames.
The RTS/CTS function allows an access point to control use of the wireless medium by a radio NIC that has activated the RTS/CTS function. The radio NIC activates the RTS/CTS function by sending a RTS frame to access point before sending a data frame. The access point may respond with a CTS frame, indicating that the radio NIC can send the data frame. In the CTS frame, the access point provides a value in a duration field of the frame header for which it will hold off other stations from transmitting while the radio NIC initiating the RTS/CTS function sends its data frame.
The PSM function allows a user to turn off a radio NIC to conserve battery power when there is no need to send data. With PSM activated, the radio NIC indicates its desire to enter “sleep” state to the access point via a status bit located in the header of a transmitted data frame. The access point takes note of each radio NIC wishing to enter a sleep state, and buffers data packets intended for the sleeping radio NICs.
In order to still receive data frames, each sleeping NIC must wake up periodically (at the right time) to receive regular beacon transmissions coming from the access point. These beacons identify whether sleeping stations have frames buffered at the access point awaiting delivery to their respective destinations. A radio NIC having awaiting buffered frames will request them from the access point. After receiving the frames, the radio NIC can go back to sleep.
In addition to providing data communications, a WLAN also can be used to physically locate the radio NIC. The use of data communications systems to perform location functions for locating mobile units is described in articles entitled Enhancements to the User Location and Tracking System, by Paramvir Bahl and Venkata N. Padmanabhan, and User Location and Tracking in an In-Building Radio Network, Microsoft Technical Report MSR-TR-99-12, dated February 1999, by the same authors, both published by Microsoft Research. As described therein signal strength of signals of the wireless data communications system, such as a system using the protocol of IEEE Standard 802.11, are used for locating mobile units within an area serviced by the system. Other techniques for locating mobile units using the wireless data communications systems or other location system are possible.
For some applications, a Real Time Location System (RTLS), which functions independently from a WLAN, may be provided for mobile units (e.g., radios or radio NICs) that are used for data communications over the WLAN. For such applications, the mobile units are provided with radio transmitters or tags, which periodically transmit identifying radio signals that are used for real time location determination. However, in these applications, the identifying radio transmissions from the mobile units can interfere with the operation of the WLAN, in that the radio signals intended for the RTLS may overwhelm the WLAN receivers, or in that that mobile units may use common power amplifiers and/or antennas for the RTLS and WLAN transmitters.
It is an object of the invention to provide arrangements and methods for mitigating interference in a mobile unit between operation of a location system transmitter and a local area network radio.